Time-resolved two-dimensional X-ray densitometry of a two-phase flow downstream of a ventilated cavity

Mäkiharju, Simo A.; Gabillet, Céline; Paik, Bu‐Geun; Chang, Natasha A.; Perlin, Marc; Ceccio, Steven L.

doi:10.1007/s00348-013-1561-z

Cited by 76 publications

(54 citation statements)

References 27 publications

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“…Cinemagraphic X-ray densitometry system A cinematographic X-ray densitometry system was used to measure the spatial distribution of the void fraction for the cavitating flow around a wedge. A complete description of the system is provided by Mäkiharju (2012) and Mäkiharju et al (2013); a brief description is provided here. The X-ray densitometry system had a source capable of 433 mA at 150 kV and the imaging system is comprised of an image intensifier coupled with a high-speed camera (Vision Research Phantom V9.0).…”

Section: High-speed Video Set-upmentioning

confidence: 99%

“…The spatial resolution of the X-ray system is 0.25 mm for the averaged void-fraction flow field, and the recordings are made at the rate of 1000 f.p.s. A more comprehensive discussion on the technique, its uncertainty and the potential sources for error was provided by Mäkiharju et al (2013). The system works on the principle of Beer-Lambert's law of attenuation of ionised radiation based on the composition of the attenuator.…”

Section: High-speed Video Set-upmentioning

confidence: 99%

“…They reported maximum averaged void-fraction values of close to 60 %, with instantaneous values exceeding 85 %. Mäkiharju et al (2013) recently developed an X-ray densitometry system that measures the time-resolved two-dimensional projection of the void-fraction flow field of gas-liquid flows, and this system was utilized for the present study. Time-resolved measurements of void-fraction flow fields can provide information about various vapour generation and destruction processes that play a key role in the onset of periodic cloud shedding.…”

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confidence: 99%

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Bubbly shock propagation as a mechanism for sheet-to-cloud transition of partial cavities

2016

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Partial cavitation in the separated region forming from the apex of a wedge is examined to reveal the flow mechanism responsible for the transition from stable sheet cavity to periodically shedding cloud cavitation. High-speed visualization and time-resolved X-ray densitometry measurements are used to examine the cavity dynamics, including the time-resolved void-fraction fields within the cavity. The experimentally observed time-averaged void-fraction profiles are compared to an analytical model employing free-streamline theory. From the instantaneous void-fraction flow fields, two distinct shedding mechanisms are identified. The classically described re-entrant flow in the cavity closure is confirmed as a mechanism for vapour entrainment and detachment that leads to intermittent shedding of smaller-scale cavities. But, with a sufficient reduction in cavitation number, large-scale periodic cloud shedding is associated with the formation and propagation of a bubbly shock within the high void-fraction bubbly mixture in the separated cavity flow. When the shock front impinges on flow at the wedge apex, a large cloud is pinched off. For periodic shedding, the speed of the front in the laboratory frame is of the order of half the free-stream speed. The features of the observed condensation shocks are related to the average and dynamic pressure and void fraction using classical one-dimensional jump conditions. The sound speed of the bubbly mixture is estimated to determine the Mach number of the cavity flow. The transition from intermittent to transitional to strongly periodic shedding occurs when the average Mach number of the cavity flow exceeds that required for the generation of strong shocks.

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Section: High-speed Video Set-upmentioning

confidence: 99%

Section: High-speed Video Set-upmentioning

confidence: 99%

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confidence: 99%

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Bubbly shock propagation as a mechanism for sheet-to-cloud transition of partial cavities

2016

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“…The former, which had been observed previously using visualisation techniques, was an expected behaviour; however, the latter, which had not been captured by conventional visualisation techniques, was an unexpected flow feature not previously observed. Similarly, a highly tuned medical X-ray source was used in Ganesh et al (2015), Mäkiharju et al (2013) and Sun et al (2015) to study cavitation in the wake of bluff bodies and two-dimensional wedges inside a water tunnel. As demonstrated, low void fraction regions in partial cavities could provide conditions necessary for the existence of bubbly shocks, which act as a dominant mechanism of shedding of partial cavities under certain conditions.…”

Section: Introductionmentioning

confidence: 99%

Application of X-ray micro-computed tomography on high-speed cavitating diesel fuel flows

et al. 2016

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efficiency of such engines. Among others, the fuel injection equipment (FIE) is a key component for improving the efficiency of environmentally friendly engines. Recent advances dictate the use of extra high injection pressures, of the order of 3000 bar and tiny flow passages (injection holes) of the order of 100-250 μm in diameter, for improved fuel atomisation and combustion efficiency. Under these circumstances, cavitation has been found to develop inside fuel injection nozzles at the early studies of Badock et al. (1999), Bergwerk (1959), Chaves et al. (1995) and Nurick (1976), followed by Arcoumanis et al. (2000), Blessing et al. (2003), Mitroglou et al. (2014) and Roth et al. (2002) in more realistic real-size nozzle geometries offering optical access; equally helpful studies performed in transparent enlarged nozzle replicas [selectively (Andriotis et al. 2008;Arcoumanis et al. 2000;Miranda et al. 2003; Mitroglou and Gavaises 2013;Powell et al. 2000)] also indicate that cavitation plays an increasingly significant role in the nozzle's internal flow structure and development. Cavitation inside an injection hole is believed to enhance spray atomisation, either directly through the implosion of cavitation bubbles or indirectly because it increases turbulence in the nozzle flow (Badock et al. 1999;Walther 2002); unfortunately, under certain circumstances induces erosion (Dular and Petkovšek 2015;Koukouvinis et al. 2016) that may lead to catastrophic failures. Moreover, cavitation inside the injection holes promotes shot-toshot spray instabilities (Mitroglou et al. 2011(Mitroglou et al. , 2012Suh and Lee 2008), which, in turn, are responsible for poor combustion efficiency and increased emissions (Hayashi et al. 2013). Two distinct macroscopic forms of cavitation have been identified, which are referred to as geometryinduced and vortex or 'string' cavitation. The former can be, partially or totally, suppressed by appropriate design of the inlet hole curvature and non-cylindrical injection hole Abstract The flow inside a purpose built enlarged singleorifice nozzle replica is quantified using time-averaged X-ray micro-computed tomography (micro-CT) and highspeed shadowgraphy. Results have been obtained at Reynolds and cavitation numbers similar to those of real-size injectors. Good agreement for the cavitation extent inside the orifice is found between the micro-CT and the corresponding temporal mean 2D cavitation image, as captured by the high-speed camera. However, the internal 3D structure of the developing cavitation cloud reveals a hollow vapour cloud ring formed at the hole entrance and extending only at the lower part of the hole due to the asymmetric flow entry. Moreover, the cavitation volume fraction exhibits a significant gradient along the orifice volume. The cavitation number and the needle valve lift seem to be the most influential operating parameters, while the Reynolds number seems to have only small effect for the range of values tested. Overall, the study demonstrates that use of micro-CT can ...

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“…However, even with such a highly sophisticated experimental technique, it renders it very difficult to measure cavitation structures or the actual void fraction in flows with cavitation. Hence, in recent years, radiological methods have become increasingly common to investigate multiphase flows such as flows with cavitation in nozzles [2,[12][13][14], pipes [15], pumps [16] and obstacles [17][18][19][20]. The reason for choosing these techniques lays in the fact, that with radiation of higher energy, scattering effects at the phase boundary between the liquid and the gas can be neglected.…”

Section: Introductionmentioning

confidence: 99%

High-speed x-ray CT imaging of a strongly cavitating nozzle flow

2018

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Examining or imaging of internal structures in flows with cavitation is still one of the greatest challenges in this field of research. In a specially designed nozzle, a strong cavitation region (CR) is generated with a liquid core (LC) in the center, surrounded by vapour. While it is almost not feasible to visualize the inside of the CR with visible light, it is shown that with high-speed x-ray computed tomography it is possible to visualize the dynamics of the unsteady cavitational flow structures inside the CR. The observed phenomena start with a ring of cavitation and small amounts of vapour inside the LC at / near the entrance of the cavitation channel. Further downstream the cavitation is growing rapidly and cavitation structures inside the LC can be identified. In addition the results are compared with former time averaged CT images of the same nozzle and with results obtained from high-speed videography.

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Time-resolved two-dimensional X-ray densitometry of a two-phase flow downstream of a ventilated cavity

Cited by 76 publications

References 27 publications

Bubbly shock propagation as a mechanism for sheet-to-cloud transition of partial cavities

Bubbly shock propagation as a mechanism for sheet-to-cloud transition of partial cavities

Application of X-ray micro-computed tomography on high-speed cavitating diesel fuel flows

High-speed x-ray CT imaging of a strongly cavitating nozzle flow

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